Silicon carbide filaments and method

ABSTRACT

A refractory substrate, which generally is graphite or carbon is overcoated with silicon carbide by chemical vapor deposition from gaseous sources of silicon and carbon. The deposition generally takes place in combination with hydrogen and the coating on the substrate generally has a thickness at least equal to the diameter of the substrate itself. A silicon carbide filament containing an inner and outer surface layer of carbon rich silicon carbide, together with a method of making the same, is described.

BACKGROUND OF THE INVENTION

This is a division of application Ser. No. 646,029 filed Jan. 2, 1976,now U.S. Pat. No. 4,068,037.

Composite plastic and metal matrix materials reinforced withhigh-modulus, high-strength filaments such as boron and silicon carbideare finding increased popularity in structural applications. Inparticular, these types of composites are useful where high strength andstiffness with accompanying low weight is desired.

The classical silicon carbide filament contains a refractory core,generally tungsten. The core is overcoated with silicon carbide. Theovercoating is accomplished by means of a hydrogen reduction chemicalvapor deposition process wherein gases containing sources of silicon andcarbon are decomposed and silicon carbide coats the core. The thicknessof the coating is directly related to the deposition time.

The most widely-used cores are tungsten. Cores of carbonaceous nature,such as graphite and carbon monofilament are being developed ascarbonaceous cores in combination with silicon carbide coatings haveexhibited greater strength and stiffness than similar coatings ontungsten cores. Accordingly, the trend has been to improve the qualityand manufacturing techniques for silicon carbide on carbon filaments.

Of the tests used to evaluate the quality of a silicon carbidemonofilament, two--the pull test and the bend test--are mostsignificant. In the pull test, opposite ends of a length of siliconcarbide filament are clamped within the jaws of a standard tensiletester and tension is applied till the filament ruptures.

In the bend test, a length of silicon carbide filament is bent aroundthe surface of a circular cylinder or disc. The stress at the outersurface of a filament is inversely proportional to the bending radiusformed by the cylinder. The maximum surface strength is determined bythe smallest diameter loop the filament can withstand without rupturing.

Standard, or non-treated, 5.6 mil silicon carbide filament on a carboncore can be bent to a minimum diameter of about 9/16th of an inch. Thiscomputes to a maximum surface strength of about 650Ksi. In pull tests,such standard silicon carbide filaments have a tensile strength of about350Ksi.

A buffer layer of oriented graphite between the core and silicon carbidecoating had no beneficial effect.

Silicon carbide filament has been shown to be sensitive to surfaceabrasion which lowers its tensile strength. In order to improve thissurface tensile strength and lessen the sensitivity to surface abrasion,a surface layer of carbon rich silicon carbide is applied to the siliconcarbide coating during the deposition process. A surface treated 5.6 milsilicon carbide filament exhibited surface tensile strength, in the bendtest, to values in the range of 1,400 to 1,600Ksi. In pull tests, thesefilaments still exhibited strength of about 350Ksi.

While a tremendous increase in surface strength was achieved by surfacetreatment, surface treatment did little for pull strength.

It is an object of the invention to provide an improved silicon carbonfilament which affords greater reliability of obtaining a higher tensilestrength than was available from prior silicon carbide filaments.

Another object of the invention is to provide a method for economicallyand reliably making improved silicon carbide filament.

In accordance with the invention, there is provided a silicon carbidefilament containing a carbon core overcoated with a coating of siliconcarbide with an inner surface layer of carbon rich silicon carbide. Thecoating may also contain an outer surface layer of carbon rich siliconcarbide.

The novel features that are considered characteristic of the inventionare set forth in the appended claims; the invention itself, however,both as to its organization and method of operation, together withadditional objects and advantages thereof, will best be understood fromthe following description of a specific embodiment, when read inconjunction with the accompanying drawings, in which:

It is hypothesized that silicon carbide is particularly sensitive to thepresence of non-stoichiometric silicon carbide or impurities. I. T.Kendall, Journal of Chemical Physics, Vol. 21, pg. 821 (1953). Sinceboth Kendall and K. Arnt & E. Hausmanne in Zeits Anorg Chem., Vol. 215,pg. 66 (1933) have found no evidence of non-stoichiometric siliconcarbide, it is hypothesized that the excess carbon appears in thesilicon carbide as an impurity. The properties of silicon carbide areparticularly sensitive to the presence of impurities such as carbon.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic representation of a reactor for making siliconcarbide filament; and

FIG. 2 is a cross-sectional representation of a silicon carbide filamentembodying the principles of the present invention.

DESCRIPTION OF THE INVENTION

FIG. 1 shows a schematic diagram 10 of a reactor for making siliconcarbide filament. The reactor 10 comprises a generally closed tubularcylinder 11 containing a pair of oppositely disposed closed ends 12 and14. A central aperture containing mercury contacts 16 and 18 are definedin each of the ends 12 and 14. The mercury contacts are coupled throughterminals a-a to a source of electrical power not otherwise shown. Therefractory or carbon core 15 is obtained from a supply reel 20. The corepasses into the cylinder 11 through the mercury contacts 16 and out ofthe cylinder through the mercury contact 18 to a take-up reel 22.

A number of ports through which gas is fed to the cylinder 11 orexhausted from cylinder 11 are provided. These will be discussed indetail hereinafter.

Briefly, the carbon core 15 in raised to a deposition temperature bymeans of electrical resistance heating in a conventional way. Typically,a mixture of hydrogen and silanes are fed into the tubular cylinder 11.When the silanes come in contact with the heated core, a chemical vapordeposition process takes place, and silicon carbide is deposited on thecore. The thickness of the silicon carbide deposited coating is afunction of the deposition temperature and the time it takes for thefilament to pass through the tubular cylinder 11.

Deposition temperatures in the range of 1,200°-1,500° C. are used. Thereactive gases comprise a blend of silanes in hydrogen. Preferred is afeedstock blend of dimethyldichlorosilane and methyldichlorosilane. Theproportions in the blend vary. In fact, silicon carbide can be depositedfrom either silane above. Additionally, it is economically advantageousto recycle the products of the reaction that are exhausted by blendingwith the original feedstock.

The point to be made is that the invention is directed to a treatmentapplied to the SiC coating made by chemical vapor deposition means. Inshort, the coating is modified.

Referring to FIG. 2, the inventive concept is to construct an innersurface layer 30 of carbon rich silicon carbide at the interface of thecarbon core 15 and the silicon carbide coating 32. The filament was alsotreated to produce an outer surface layer 34 of carbon rich siliconcarbide. When this is done, particularly in the manner described below,silicon carbide on carbon filament with consistent pull strengths of600-800Ksi and bend strengths of 1,400-1,600Ksi are made at economicalproduction rates. Silicon carbide fibers having different diameters wereproduced. Optimum results were achieved when the diameter of the carbonrich region was one-half of the overall diameter. A silicon carbidecoating which was uniformly carbon rich was very weak and generallyunsatisfactory.

The reasons for the improvement are not clear. While the outer surfacelayer treatment 34 was very thin and certainly less than 0.1 mil, theinner surface layer treatment at its optimum measured 0.65 mil inthickness for a 5.6 mil filament and was useful in thicknesses of about0.35-1.5 mils, depending on the overall diameter of the filament.Interestingly, if the entire cross-section of the silicon carbidecoating is carbon rich, the filament is very weak.

In general, the thickness of the inner surface layer is 30-60% of thethickness of the silicon carbide coating.

The outer surface layer was produced for the purpose of reducingsensitivity to surface abrasion. This factor does not knowingly appearpertinent at the inner surface of silicon carbide coating. The innersurface layer is constructed by combining with the silane and hydrogenfeedstock a blend of argon and a hydrocarbon, the latter preferablybeing propane at the top of the reactor through port 24. The carbon richsilicon carbide outer surface layer is produced differently.

At some distance d₁ below port 24, the mixture is diluted by additionalhydrogen and silane and, at times, nitrogen and air through port 26. Thediluted mixture is exhausted through port 28.

The deposition temperature in the region between ports 24 and 26 ishigher than normally used below port 26 and is in the range of1,400°-1,500° C. for propane. This high temperature may be maintained ina number of ways. This is the primary purpose of the argon. Localizedr-f heating may also be used.

Note, measurement difficulties create an uncertainty of about 100° C. inall indicated temperatures.

The temperature is lowered to between 1,300° and 1,400° C. below theport 26 and may decrease to about 1,200° C. just above port 25. Theouter surface layer is produced by introducing argon and propane throughport 29. The temperature at the lower end of the cylinder 11 betweenports 28 and 29 is maintained in the range of 1,300°-1,400° C. Highertemperatures destroy the strength of the filament. Lower temperaturesare ineffective.

Hydrocarbons are the best sources of carbon to enrich the siliconcarbide. Propane and butane were very effective. While methane did notwork at the bottom of the reactor because of the low temperature, it maybe effective at the top. Isobutane and cyclobutane are also recommended.

Nor is the process limited to gases. Benzene, gasoline and hexane haveproven useful in the past for chemical vapor desposition of carbon onheated substrates.

In short, any substances which can be pyrolitically dissociated at thedeposition temperatures quoted should work.

To make silicon carbide filament pursuant to the invention, the carboncore is prepared in a conventional way and fed to the reactor 10 asindicated in FIG. 1. At the top of the reactor at port 24, silane blend,hydrogen, argon and propane are fed to the reactor in quantities todeposit on the core 15 a carbon rich silicon carbide layer. Somenitrogen and air is also added. At a distance d₁ below port 24,additional silane blend and hydrogen are added to dilute the mixture ofgases in contact with the core 15 below the port 26.

Between the ports 24 and 26, the core 15 temperature is high and in therange of 1,400°-1,500° C. Normal deposition temperatures, in the rangeof 1,200°-1,350° C. are maintained below port 26.

The gases are exhausted at port 28. The filament may be surface treatedby adding propane and argon through port 29 with the depositiontemperature in the range 1,300°-1,400° C.

The following conditions are maintained for an 8-foot-long reactorhaving an internal diameter of about 0.75 inches and d₁ = 7 inches. Coretravel is 15-20 ft/min.

    ______________________________________                                        GAS INTO PORT 24                                                              0.65 liters/min                                                                          silane blend                                                                              ##STR1##                                                                      ##STR2##                                               0.24 liters/min                                                                          hydrogen                                                           0.06-0.3 liters/min                                                                      Argon*                                                             0.1 liters/min                                                                           Propane*                                                           0.18 liters/min                                                                          Nitrogen and Air                                                                          ##STR3##                                                *Nitrogen and air cmprise 3-5% of hydrogen. Propane and argon comprise        1-10% of (hydrogen or silane and hydrogen).                              

     -                                                                            GAS INTO PORT 26                                                              2.7 liters/min silane blend                                                   4.8 liters/min hydrogen                                                       .19 liter/min nitrogen and air*                                               GAS INTO PORT 29                                                              0.04 liters/min propane                                                       0.16 liters/min argon                                                         PRODUCT                                                                       Carbon core Dia      1.3 mil                                                  Silicon carbide filament Dia                                                                       5.6 mils                                                 Inner surface layer thickness                                                                      0.65 mil                                                 Outer surface layer thickness                                                                      less than 0.1 mil                                        Pull strength        600-800ksi                                               Bend strength        1,400-1,600ksi                                           ______________________________________                                    

The argon is added primarily to raise the temperature of the core. Inall probability, it may be eliminated if supplemental r-f heating isused.

It must be emphasized that the basic concept relates to the constructionof a carbon rich silicon carbide inner surface layer for improving thestrength of silicon carbide filaments.

The process parameters can be varies. Departures from the ratiosexpressed above may be compensated for by varying one or more otherparameters.

The various features and advantages of the invention are thought to beclear from the foregoing description. Various other features andadvantages not specifically enumerated will undoubtedly occur to thoseversed in the art, as likewise will many variations and modifications ofthe preferred embodiment illustrated, all of which may be achievedwithout departing from the spirit and scope of the invention as definedby the following claims.

We claim:
 1. A method of making high-strength silicon carbide filamentin a continuous process comprising the steps of:passing a carbonaceousfilament through a reactor; heating the filament to a temperature of1400°-1500° C.; exposing the filament while at an elevated temperatureto a mixture consisting essentially of a blend of dimethyldichlorosilaneand monomethyldichlorosilane, hydrogen and a substance capable ofreleasing carbon when heated for forming a carbon rich silicon carbidecoating on the filament; adding additional silane blend and hydrogen tosaid mixture to lower the temperature of said filament; and exposing thecarbon rich silicon carbide coating to the mixture with said addedsilane and hydrogen for effecting the deposition of a silicon carbidecoating on said carbon rich silicon carbide layer.
 2. A method asdescribed in claim 1 wherein said silicon carbide coating is formed at1200°-1400° C.
 3. A method as described in claim 2 wherein in additionan outer coating of carbon rich silicon carbide is deposited on thesilicon carbide coating by raising the temperature of said filament andadding a source of carbon to said mixture.
 4. A method of making ahigh-strength silicon carbide filament in a continuous processcomprising the steps of:passing a carbonaceous filament through atubular reactor; exposing the filament while at an elevated temperatureof 1400°-1500° C. to a mixture consisting essentially of a blend ofdimethyldichlorisilane and monomethyldichlorisilane, hydrogen, argon,and a substance capable of releasing carbon when heated for depositing acarbon rich silicon carbide layer on the filament; diluting said argonand carbon in said mixture with additional silane and hydrogen forlowering the temperature of said filament; and exposing the carbon richsilicon carbide layer to said diluted mixture at said lower temperaturefor effecting the deposition of a silicon carbide coating on said carbonrich silicon carbide layer.
 5. A method as described in claim 4 whereinthe diluent includes also nitrogen and air.
 6. A method as described inclaim 4 wherein said silicon carbide coating is formed at 1200°-1400° C.7. A method as described in claim 4 wherein additional quantities ofargon and a source of carbon are added to decrease the heat loss fromsaid filament and for depositing a second carbon rich silicon carbidecoating on said filament, respectively.